Plant production and water purification efficiency by rice and umbrella plants grown in a floating culture system under various water environmental conditions

The floating culture system was originally designed with a purpose of devel oping a new cropping area by growing plants on the water surface; in additi on, this system can also be used as a technique for water purification by a llowing plants to absorb nutrients from the eutrophied water, We investigat ed here the specific differences in water purification effect and plant pro ductivity of rice and umbrella plants both of which were grown on the surfa ce of the waters with various levels of eutrophication. The results were as follows: 1) In both species, the rate of nitrogen absorption (NR) was posi tively related to the crop growth rate (CGR). Both parameters greatly chang ed in value with water conditions, showing a negative relation with pH and positive relation with electrical conductivity (EC). 2) Application of a sl ow-release fertilizer as basal-dressing to rice in the eary growth stage ha d a beneficial effect on subsequent growth vigor, and the highest grain yie ld of rice was 542 g m(-2) comparable to the yield obtained in paddy-growm plants, The leaf photosynthetic rate, chlorophyll content and plant weight were increased, depending on the water quality, by applying acid and iron c ontrolling chemicals together with the basal-dressing fertilizer. High pH c aused an iron deficiency in rice and greatly restricted its growth. In the eutrophied waters, iron fertilizer application to plants was very effective in improving the growth. 3) CGR and water purification effect of umbrella plants were significantly superior to those of rice examined under all of t he growth conditions in this study. The water purification by umbrella plan ts is practically expected in eutrophied water areas.